To support broadband wireless (e.g., WiMAX) access, there are different types of broadband wireless air interfaces, such as cellular 3G technologies (e.g., UMTS, WCDMA, etc.), and multi-carrier based multiple access techniques (e.g., OFDMA, OFDM-TDMA, OFDM-CDMA, etc.). Frequency division multiplexing involves sub-channelization, of which at least four types (OFDM, Flash OFDM, sOFDMA and OFDMA) exist.
Orthogonal Frequency Division Multiplexing (OFDM) involves the splitting of a radio signal into multiple smaller sub-signals that are then transmitted simultaneously at different frequencies to a receiver. OFDM refers to a form of multi-carrier transmission where all the sub-carriers are orthogonal to each other. Certain IEEE standards and 3GPP standards are related to various aspects of OFDM.
FIGS. 1 and 2 show a typical frame that is used in OFDM. One frame has a time duration of 10 ms (milliseconds) and consists of 20 sub-frames, each having a time duration of 0.5 ms. Each sub-frame may consist of a resource block (RB) that contains data or information, and a cyclic prefix (CP) that is a guard interval needed for conventional OFDM modulation (but not needed for OFDM with pulse shaping, i.e., OFDM/OQAM). The sub-frame duration corresponds to the minimum downlink TTI (Transmission Time Interval).
FIG. 3 shows a basic downlink reference-signal structure consisting of known reference symbols. Namely, a mapping of physical channel symbols in frequency domain is shown. In other words, channel-coded, interleaved, and data-modulated information (i.e., Layer 3 information) is mapped onto OFDM time/frequency symbols. The OFDM symbols can be organized into a number (M) of consecutive sub-carriers for a number (N) of consecutive OFDM symbols.
Here, it is assumed that 7 OFDM symbols exist per sub-frame (when the CP length is short). In case of a long CP or a different frame structure, this basic downlink reference-signal structure would be slightly different.
Reference symbols (i.e., first reference symbols) are located in the first OFDM symbol of every sub-frame assigned for downlink transmission. This is valid for both FDD and TDD, as well as for both long and short CP. Additional reference symbols (i.e., second reference symbols) are located in the third last OFDM symbol of every sub-frame assigned for downlink transmission. This is the baseline for both FDD and TDD, as well as for both long and short CP. However, for FDD, an evaluation of whether the second reference symbols are need should be made.
FIG. 4 shows an exemplary structure of an Evolved Universal Mobile Telecommunications System (E-UMTS). The E-UMTS system is a system that has evolved from the UMTS system, and its standardization work is currently being performed by the 3GPP standards organization.
The E-UMTS network generally comprises at least one mobile terminal (i.e., user equipment: UE), base stations (i.e., Node Bs), a control plane server (CPS) that performs radio (wireless) control functions, a radio resource management (RRM) entity that performs radio resource management functions, a mobility management entity (MME) that performs mobility management functions for a mobile terminal, and an access gateway (AG) that is located at an end of the E-UMTS network and connects with one or more external networks. Here, it can be understood that the particular names of the various network entities are not limited to those mentioned above.
The various layers of the radio interface protocol between the mobile terminal and the network may be divided into L1 (Layer 1), L2 (Layer 2), and L3 (Layer 3) based upon the lower three layers of the Open System Interconnection (OSI) standard model that is known the field of communication systems. Among these layers, a physical layer that is part of Layer 1 provides an information transfer service using a physical channel, while a Radio Resource Control (RRC) layer located in Layer 3 performs the function of controlling radio resources between the mobile terminal and the network. To do so, the RRC layer exchanges RRC messages between the mobile terminal and the network. The functions of the RRC layer may be distributed among and performed within the Node B, the CPS/RRM and/or the MME.
FIGS. 5 and 6 show an exemplary architecture of the radio interface protocol between the mobile terminal and the UTRAN (UMTS Terrestrial Radio Access Network). The radio interface protocol of FIGS. 5 and 6 is horizontally comprised of a physical layer, a data link layer, and a network layer, and vertically comprised of a user plane for transmitting user data and a control plane for transferring control signaling. The radio interface protocol layer of FIGS. 5 and 6 may be divided into L1 (Layer 1), L2 (Layer 2), and L3 (Layer 3) based upon the lower three layers of the Open System Interconnection (OSI) standards model that is known the field of communication systems.
Particular layers of the radio protocol control plane of FIG. 5 and of the radio protocol user plane of FIG. 6 will be described below. The physical layer (i.e., Layer 1) uses a physical channel to provide an information transfer service to a higher layer. The physical layer is connected with a medium access control (MAC) layer located thereabove via a transport channel, and data is transferred between the physical layer and the MAC layer via the transport channel. Also, between respectively different physical layers, namely, between the respective physical layers of the transmitting side (transmitter) and the receiving side (receiver), data is transferred via a physical channel.
The MAC layer of Layer 2 provides services to a radio link control (RLC) layer (which is a higher layer) via a logical channel. The RLC layer of Layer 2 supports the transmission of data with reliability. It should be noted that the RLC layer in FIGS. 5 and 6 is depicted in dotted lines, because if the RLC functions are implemented in and performed by the MAC layer, the RLC layer itself may not need to exist. The PDCP layer of Layer 2 performs a header compression function that reduces unnecessary control information such that data being transmitted by employing Internet protocol (IP) packets, such as IPv4 or IPv6, can be efficiently sent over a radio (wireless) interface that has a relatively small bandwidth.
The radio resource control (RRC) layer located at the lowermost portion of Layer 3 is only defined in the control plane, and handles the control of logical channels, transport channels, and physical channels with respect to the configuration, reconfiguration and release of radio bearers (RB). Here, the RB refers to a service that is provided by Layer 2 for data transfer between the mobile terminal and the UTRAN.
As for channels used in downlink transmission for transmitting data from the network to the mobile terminal, there is a broadcast channel (BCH) used for transmitting system information, and a shared channel (SCH) used for transmitting user traffic or control messages. As for channels used in uplink transmission for transmitting data from the mobile terminal to the network, there is a random access channel (RACH) used for transmitting an initial control message, and a shared channel (SCH) used for transmitting user traffic or control messages.